1. Field of the Invention
The present invention relates generally to apparatus, systems and methods for wireless communication. More particularly, the present invention relates to apparatuses, systems and methods for modulation in an impulse radio communications system. The present invention also relates to apparatuses, systems and methods for transmitting and receiving modulated impulse radio signals.
2. Related Art
The radio transmission of both analog and digital communications intelligence has normally been effected by one of two methods. In one, referred to as an amplitude modulation, a continuous sinusoidal radio frequency carrier is modulated in amplitude according to an intelligence or communications signal. When the amplitude modulated signal is received at a receiving location, the reverse process (that is, demodulation of the carrier) is effected to recover the intelligence. The other method employs what is termed frequency modulation. In frequency modulation, instead of amplitude modulation of the carrier signal, the carrier signal is frequency modulated according to the intelligence. When a frequency modulated signal is received, circuitry is employed which performs what is termed discrimination wherein changes in frequency are changed to changes in amplitude in accordance with the original modulation, and thereby a communications signal is recovered. In both systems a continuous sinusoidal carrier is assigned to and occupies a distinctive frequency band width, or channel. In turn, this channel occupies spectrum space which, if interference is to be avoided, cannot be utilized by other transmissions.
Today almost every nook and cranny of spectrum space (also referred to as the frequency spectrum) is being utilized. According, there is a tremendous need for some method of expanding the availability of medium for communications. In consideration of this, new methods and systems of communications have been developed that employ a wider frequency spectrum, rather than discrete frequency channels, for radio communications links. More specifically, new methods and systems of communications have been developed that utilize wide band or ultra wide band (UWB) technology, which is also called impulse radio communications.
Impulse radio communications was first fully described in a series of patents, including U.S. Pat. No. 4,641,317 (issued Feb. 3, 1987), U.S. Pat. No. 4,813,057 (issued Mar. 14, 1989), U.S. Pat. No. 4,979,186 (issued Dec. 18, 1990) and U.S. Pat. No. 5,363,108 (issued Nov. 8, 1994) to Larry W. Fullerton. A second generation of impulse radio patents include U.S. Pat. No. 5,677,927 (issued Oct. 14, 1997), U.S. Pat. No. 5,687,169 (issued Nov. 11, 1997) and U.S. Pat. No. 5,832,035 (issued Nov. 3, 1998) to Fullerton et al. Each of these patent documents are incorporated herein by reference.
Basic impulse radio transmitters emit short pulses approaching a Gaussian monocycle with tightly controlled pulse-to-pulse intervals. Impulse radio systems typically use pulse position modulation (also referred to as digital time shift modulation), which is a form of time modulation where the value of each instantaneous sample of a modulating signal is caused to modulate the position of a pulse in time. More specifically, in pulse position modulation, the pulse-to-pulse interval is typically varied on a pulse-by-pulse basis by two components: a pseudo-random code component and an information component. That is, when coding is used each pulse is shifted by a coding amount, and information modulation is accomplished by shifting the coded time position by an additional amount (that is, in addition to PN code dither) in response to an information signal. This additional amount (that is, the information modulation dither) is typically very small relative to the PN code shift. For example, in a 10 mega pulse per second (Mpps) system with a center frequency of 2 GHz, the PN code may command pulse position variations over a range of 100 nsec; whereas, the information modulation may only deviate the pulse position by 150 ps (which is typically less then the wavelength of a pulse).
Although the above described information modulation scheme has proved effective for certain applications, there is a desire to create information modulation schemes that increase data throughput and/or decrease the probability of bit errors. Further, there is a desire to create modulation schemes that exploit the unique aspects of impulse radio communications.